Hot-isostatic pressing is used, for example, for correcting defects such as cracks, pores or other voids in metallic materials. The treatment is especially valuable for removing defects in parts of expensive material, for example gas-turbine parts such as turbine blades of titanium or other so-called superalloys.
The hot-isostatic pressing is usually carried out in a pressure chamber with an inert gas as pressure medium and at a high temperature. Impurities in the gas, such as oxygen, nitrogen or water steam, have a very harmful effect on such materials as superalloys and can deteriorate the strength or toughness in a destructive manner, or form coatings which have to be removed by work operations causing material loss and high costs. To maintain the content of impurities in the inert gas low, it is known to place purifying substances in the pressure chamber for which the impurities have a greater affinity than for the parts which are to be treated. The affinity of the impurities for a substance depends partly on material properties, partly on the temperature of the substance. To increase the affinity, it is therefore possible to increase the temperature. The purifying substances may consist of aluminium, titanium or zirconium, or of alloys containing these substances. A large contact surface is desirable, and therefore the purifying substances are suitably in the form of chips, grains or powder.
Swedish patent application 7614549-9 describes one method for hot-isostatic treatment by means of an inert gas and in the presence of a purifying agent for the inert gas. According to this method, the parts which are to be treated are placed inside a container in the form of a cylinder. At the upper and lower parts of the cylinder, there are openings communicating with an annular space which is arranged between the cylinder and a cover arranged outside of this. At the top of the cylinder there is a basket with a perforated bottom. This basket accommodates the purifying agent in order to allow axial flow of the gas. During the treatment, the cylinder with the surrounding cover is placed inside a furnace arranged in a pressure chamber. To achieve pressure equalization between the spaces outside and inside the cover, the gas may pass under the lower edge of the cover.
During the treatment, heating elements in the furnace chamber are activated. The heat is transferred via the cover to the gap between the cover and the cylinder. The gas which is present in the gap is heated, causing the gas inside the cover to start circulating. During the circulation, the gas rises upwards in the gap and thereafter passes through the openings in the upper part of the cylinder and further axially down through the purifying agent in order thereafter to overflow the parts in the cylinder. At the lower part of the cylinder, the gas passes out through the openings and then again rises up through the gap.
Since the hot gas, after having passed through the purifying agent, directly overflows the parts in the container, the temperature of the purifying agent is essentially the same as that of the parts. If the purifying agent is of the same material as the parts, the impurities in the gas will therefore have the same tendency to react with the purifying agent as with the parts.
Recently, this has caused considerable problems during hot-isostatic treatment of modern materials. The aircraft industry, for example, is nowadays using to an increasing extent the very materials which are good purifying agents also as high-tensile construction materials for, for example, turbine blades. One of the very best materials from the point of view of strength is titanium. However, titanium is one of the materials which have the greatest tendency to react with the harmful gas impurities oxygen and nitrogen. During hot-isostatic treatment of titanium, therefore, it is not possible to find any purifying agent which is superior to titanium.
During treatment of titanium parts according to the method described above, the impurities in the gas therefore are equally prone to react with the parts as with the purifying agent. Since only some of the impurities in the gas react with and are bound by the purifying agent during each passage thereof, an unacceptably large part of the impurities will instead react with the parts. The result of the hot-isostatic treatment therefore becomes inferior and leads to a low yield and a high degree of rejection of parts. Still worse, however, is that the insufficient treatment may also cause faults in the material of the parts which are difficult to detect and which may cause very serious damage, for example if the treated parts constitute turbine blades for aircraft engines.
An additional problem has arisen lately as a consequence of the increasingly higher treatment pressures which are used during hot-isostatic treatment. When the gas is compressed during the pressurization, the concentration of the amount of impurities per unit of volume of gas increases. In this way, gas which exhibits acceptable impurity levels at atmospheric pressure thus becomes too laden with impurities when pressurized. This means that completely new gas of the highest purity, also when coming straight from the manufacturer, has too high an impurity level to be useful, in unpurified state, during hot-isostatic pressing. Thus, also completely new gas must be purified before coming into contact with the parts.
Still another closely related problem has arisen with the introduction of a controlled rapid cooling of the parts at the final state of the hot-isostatic treatment. Modern hot-isostatic presses, which are adapted for controlled rapid cooling, have one or more circulation loops for the gas arranged outside the furnace chamber. During the rapid cooling, a sub-flow of the gas is brought to pass through these loops for cooling by transfer of heat to the pressure chamber wall. However, it has proved that impurities, for example in the form of water and oxygen, adhere to the walls in these circulation loops. The impurities do not disappear entirely during the vacuum suction of the press but run the risk of being mixed with the gas during the treatment, thus damaging the parts.
The object of the present invention is, therefore, to provide a method and a device for hot-isostatic pressing of parts, which considerably reduces the risk of impurities present in the gas damaging the parts, especially in those cases where the parts contain the same material as the purifying agent.